Power HeadRoom Report in LTE

Power Headroom Report(PHR) in LTE :- 

                             In Uplink, the UE has a maximum transmission power defined by their power class. Power Headroom indicates how much relative transmission power (With respect to UE maximum transmission power) can be further used by UE. In mathematical expression, Power Headroom can be expressed as below:

Power Headroom = UE max transmission Power - Power of PUSCH channel (Estimated or Scheduled not really transmitted).

Note:- Power Headroom Report is transmitted in same subframe in which PUSCH is transmitted.

By the mathematical expression there are 2 possibilities :

1. Power Headroom is +Ve ; Means UE is still having adequate transmission power left and thus network can assign more resource blocks to UE.

2. Power Headroom is -Ve ; Means UE did not have anymore additional power left and is transmitting at more than its maximum transmitting power already. In this case network will not assign more resource blocks to UE.

Power Headroom Report Mapping :- 
     The PHR ranges from -23.. to +40dB .

When UE send PHR ?- There are two triggering condition for UE through RRC messages( RRC configuration, RRC reconfiguration).

1. When Downlink Pathloss changed is greater than a certain threshold(in dB)
2. By some predefined periodic PHR timer (in Sub frames).

Note:- PHR can be efficiently used in UL power control to improve the throughput.